The tag says read only and I did not lock it
Someone did, and it was probably not you. Tags can arrive locked from manufacture, carry a lock set by a previous owner, or be locked by a writing tool that had an option enabled by default. All three produce the same permanent state.
Three ways it happened
Locked at manufacture. Some tags are sold pre encoded and locked, particularly promotional or single purpose products. The listing may not say so.
Locked by a previous owner. Any tag that has been used before may have been locked deliberately. Second hand tags and tags recovered from a previous deployment both fall here.
Locked by a tool. Some writing applications offer a lock option, and it can be left switched on from a previous job. The tag is then locked on its first write without anyone deciding to.
The third is the most common and the most avoidable, and it is the one people do not suspect because they never chose it.
The option that locks by default
Check the settings of whatever wrote the tag. A checkbox labelled to make the tag read only, permanent, or write protected is often near the write button.
If it was on, every tag written in that session is now permanent. Check the others before assuming this one was isolated.
Turn it off and leave it off. Locking is a deliberate final step, not a default, and it belongs at the end of a deployment once the content is verified.
This is worth checking even when you are certain, because the cost of being wrong is a whole batch.
Whether it matters
A locked tag is not a broken tag. It reads perfectly and will keep reading for as long as any other tag.
So the question is whether the content on it is what you want. If it holds a stable redirect address you control, being unable to change it costs you nothing, because what it points at remains changeable.
If it holds the wrong content, or content that will need to change, the tag is finished.
Judge the tag by what it says rather than by its state. A locked tag with correct content is a working tag.
Preventing the next one
Read a sample before writing a batch, which catches factory locked and previously locked tags before you waste effort on them.
Check the lock option in your writing tool before every session, not just the first.
Write and verify the whole batch first, and lock afterwards as a separate deliberate pass. Locking during the write means a mistake is permanent the moment it is made.
And keep recovered tags separate from new stock. A drawer mixing the two produces exactly this surprise months later.
Frequently asked questions
Can a locked tag be unlocked?
No. Lock bits are a permanent hardware change, and no tool, application or manufacturer reverses them.
Do tags come locked from the factory?
Most do not, and pre encoded promotional tags sometimes do. Reading a sample on arrival settles it before you write.
Could my phone have locked it accidentally?
Not by reading, and yes by writing with a lock option enabled. Check the settings of the application that wrote it.
Is a locked tag still usable?
Entirely, if the content is right. Locking prevents changes and does nothing to reading.